Accurate positioning in wireless cellular systems is important and will become even more important when regulators are forcing operators to fulfill e.g. E911 (in the US) and E112 (in EU) emergency location requirements. In such situations it is particularly important to secure good performance in terms of availability, response time and accuracy. Furthermore, different kinds of additional services based on the position of a mobile terminal become more and more common.
In order to obtain accurate positioning information there are a number of different approaches that are available in cellular systems today. Some of them are based solely on measurements conducted within the radio network itself, while others use external sources such as satellite positioning systems to acquire the position in the cellular system. Some methods involve measurements or other actions performed by the mobile, while other methods rely on information obtained in other ways. Generally, most methods of high accuracy involve actions performed by the mobile terminal.
High precision positioning methods are in the present disclosure intended to denote positioning methods that have a potential to meet the North-American E-911 emergency positioning requirements. Methods that meet these requirements are capable of obtaining positioning accuracies of either 50 meters (in 67% of all cases) and 150 meters (in 95% of all cases) for terminal based methods, or 100 meters (in 67% of all cases) and 300 m (in 95% of all cases) in network based methods.
Positioning by solely identifying the cell in which the mobile is hosted does basically not involve the mobile terminal. However, such a cell ID positioning can be enhanced, e.g. by combining it with determinations of distance to a base station, e.g. based on round trip time measurements.
Among terminal based methods can also be mentioned e.g. assisted GPS (A-GPS) positioning, where satellite signals are utilized for positioning purposes. Cellular radio signal time differences can also be utilized, e.g. Uplink Time Difference Of Arrival (UTDOA) positioning or the downlink counterpart Observed Time Difference Of Arrival-Idle Period Down Link (OTDOA-IPDL).
In general, a procedure for positioning of a mobile terminal with a terminal based positioning method always requires the issue of a positioning order from a positioning node in the stationary part of the cellular communications system. The positioning order is directed only to the terminal to be positioned and is issued when a positioning is requested from any party within or connected to the communications system, having appropriate authority. The terminal performs the requested positioning actions and report the result in terms of a position or measurement results to a cellular system node, typically a positioning node.
A general problem with prior art positioning management is that the availability, response time and accuracy are not always satisfactory. If many terminals within a limited area are of use for positioning services at the same time, the amount of signaling resources occupied by management messages may be large, possibly resulting in lack of availability or an unsatisfactory response time. Furthermore, several of the available positioning methods are relatively time consuming, at least if high accuracy determinations are requested. There are also some limitations in possibilities of performing any positioning at all, e.g. for indoor situations.
The different positioning approaches have furthermore additional specific problems. In order to obtain good positioning performance e.g. with GPS, a substantial number of satellites need to be visible from the terminal. This makes satellite based positioning methods less suitable e.g. for indoor use. Fine time assistance systems have to be employed, which, however, typically requires additional functionality in the network.
For terrestrial methods based on downlink measurements, an obstacle is the number of detectable signals. In order to have good performance and to meet e.g. E-911 emergency positioning requirements, signals from at least 6-8 non co-located signal transmission positions need to be detected in a downlink based positioning system. This will in general, not only require long measurement times, but also good signal conditions with respect to the transmitting sites. Note that the problem is particularly difficult in code division multiple access (CDMA) systems. The reason is that all sites transmit in the same frequency band. This results in a near-far problem, where terminals close to the transmitting site of the own cell, experience extensive interference from said own site, a fact that prevents detection of signals from other more distant transmitting sites. This problem is exactly what has prevented full-scale implementation of the OTDOA-IPDL positioning method.
Similar problems exist for the uplink case, where power up may be needed in order to secure hearabilty in distant receiving sites. This occurs since in CDMA systems the transmitting power of terminals is normally controlled by the radio base station (RBS), so that all terminals in the cell are detected at comparable power levels in the RBS. Hence, terminals close to the RBS of the own cell are not likely to transmit at a power level that are sufficient for detection at distant RBS's.